1. Field of the Invention
The present invention relates to a zoom lens, and more particularly to a retrofocus zoom lens.
2. Description of the Related Art
A projector is an apparatus that projects an image formed on a personal computer, a video camera, or the like to a large screen for viewing, and is widely used in situations such as meetings, presentations, and movie watching at home. There are several demands on projectors. For example, zoom lenses (projection lenses) intended for projectors are in general desired to have sufficient lengths of back focus, to be telecentric on the reduction conjugate side thereof, and to have less lateral chromatic aberration and less distortion.
Zoom lenses intended for projectors are also desired to have shorter projector distances for the convenience of installation. Accordingly, a retrofocus lens configuration is employed in which, with respect to the stop, a front lens unit provided on the enlargement conjugate side has a negative refractive power and a rear lens unit provided on the reduction conjugate side has a positive refractive power.
A retrofocus lens, however, is asymmetric with respect to the stop and therefore often causes negative distortion and/or lateral chromatic aberration. This particularly applies to projector lenses desired to be telecentric on the reduction conjugate side. As such an asymmetric characteristic become more pronounced, the refractive powers of positive lens elements included in the rear lens unit increase. Consequently, spherical aberration and longitudinal aberration, such as longitudinal chromatic aberration, become more difficult to correct.
With the recent recognition of a problem of television (TV) distortion of the projected image, it is also desired to reduce distortion. Moreover, with a recent increase in resolution, it is desired to suppress chromatic aberration more strictly.
To correct the distortion occurring in a retrofocus lens, it is general to produce positive distortion by providing a positive lens element at a position in the front lens unit where the marginal ray height is large so that rays are converged. In such a configuration, lateral chromatic aberration is also corrected. However, since the level of correction of lateral chromatic aberration varies with the wavelength, rays of short wavelengths tend to be overcorrected.
To solve such a problem, several techniques of correcting the overcorrected lateral chromatic aberration caused by short-wavelength rays are disclosed in which an element causing a large chromatic dispersion is provided at a position in the rear lens unit where the marginal ray height is large.
In Japanese Patent Laid-Open No. 2001-100100, the final lens is made of anomalous high-dispersion glass. However, the effect of correction produced by this technique is limited. In Japanese Patents Laid-Open No. 2001-154091 and No. 2002-048974, a “chromatic” cemented lens is provided as the final lens element. Although this technique may be more effective than the technique disclosed in Japanese Patent Laid-Open No. 2001-100100, the cemented surface is large and the combination of lens elements employed therein causes spherical aberration. Therefore, even if lateral chromatic aberration is reduced, the resolution is not increased. Considering the above, the techniques disclosed in Japanese Patents Laid-Open No. 2001-100100, No. 2001-154091, and No. 2002-048974 do not provide performance sufficient for meeting the recent demands on zoom lenses intended for projectors.